3.1 Eliminate Slow or Noisy Input Signals

Hello, and welcome to The Logic Minute. In this video, we will look at how to use logic to clean up slow or noisy input signals. All standard CMOS inputs have a single input threshold value, identified here as a gray dashed line, at which the input switches between a logic low and a logic high. A noisy input can cross the threshold multiple times causing oscillations, as shown here on the red output signal and will cause added power consumption, shown with the yellow supply current measurement.
Note that each time a noise peak crosses the threshold, the output switches. And each time the output switches, there's a spike in supply current. When the output switches many times, this can cause a noticeable increase in power consumption. Obviously, these side effects are undesirable, and we want to avoid them. A slow input can produce oscillations also and will definitely produce added supply current.
Note that the operating current of this device is typically measured in nanoamps or microamps. While the current caused by a slow input is usually measured in milliamps This current can even be large enough to damage the device and is the reason why CMOS devices have a maximum input transition rate. Each CMOS device's datasheet will explicitly define a maximum input transition rate, usually measured in nanoseconds per volt, that will prevent this issue from occurring.
A Schmitt-Trigger input device can be used to clean up slow or noisy signals. This input design separates the positive going and negative going thresholds as shown here. With slow input signals, the output will switch only once and has a clean edge for any CMOS input following the Schmitt-Trigger device. The supply current will still increase as the input signal approaches the threshold. However, the input is designed to handle this and will not be damaged.
Additionally, as long as the input signal's noise amplitude does not exceed the minimum hysteresis value defined for the Schmitt-Trigger inputs, the device will have a clean output.
Thank you for watching. Please explore the other videos and training material on our site. And if you have any questions, come over to the E2E forum to ask us directly.

Description

March 28, 2018

Sometimes slow or noisy signals are unavoidable in a system. This video discusses some of the specific issues seen when connecting a slow or noisy signal into a CMOS input, and what can be done to eliminate those signals.